Microwave sterilization of pre-packaged foodstuffs has been researched for a number of years. See, for example, U.S. Pat. Nos. 4,962,298 and 5,074,200 which are directed to a microwave system for sterilization of pre-packaged food articles; U.S. Pat. No. 4,974,503 which describes and apparatus for irradiating food products to pasteurize, sterilize or uniformly heat food products; U.S. Pat. No. 4,952,763 which describes a system for sterilizing food packages with microwaves under pressure; U.S. Pat. No. 5,750,966 which describes a plant for pasteurizing or sterilizing food products using microwaves in a pressurized chamber; and U.S. Pat. No. 7,119,313 which describes a processing line for pasteurizing and/or sterilizing foodstuffs with microwaves. Microwave sterilization and pasteurization of foods is being commercially used in Europe by TOP's Foods (Tang, Microwave Heating News Vol. 2(3), 2000) where temperature monitoring of the food is measured indirectly by infrared temperature sensors and displacement sensors which monitor bulging of the tray tops. Despite the commercial success of microwave sterilization in Europe, the process has not been widely adopted in the U.S.
It would be advantageous in a large scale microwave based food sterilization or pasteurization system to provide a mechanism for assuring proper sterilization or pasteurization for different batches of product and for allowing identification of areas within the processing line which require repair or adjustment.